Reduced engine operation times in hybrid vehicles, such as plug-in hybrid vehicles, enable fuel economy and reduced fuel emissions benefits. However, the shorter engine operation times can lead to insufficient purging of fuel vapors from the vehicle's emission control system. To address this issue, hybrid vehicles may include a fuel tank isolation valve (FTIV) between a fuel tank and a hydrocarbon canister of the emission system to limit the amount of fuel vapors absorbed in the canister. Specifically, the normally closed FTIV separates storage of refueling vapors from the storage of diurnal vapors, and is opened during refueling and purging to allow refueling vapors to be directed to the canister.
During leak detection operations, the FTIV is kept closed to better enable pressures and vacuums to be generated. Leaks are then detected based on pressure changes following FTIV closure. However, if the FTIV function is degraded, inaccurate leak detection may occur.
Thus in one example, the above issue may be at least partly addressed by a method of monitoring a fuel vapor recovery system. In one example embodiment, the method comprises, modulating a fuel tank isolation valve (FTIV) coupled between a fuel tank and a canister of the fuel vapor recovery system, and indicating FTIV degradation based on pressure pulsations upstream and/or downstream of the FTIV responsive to the modulation.
In one example, during a diurnal cycle, when the engine is off and a threshold pressure difference across the FTIV has been obtained, the valve may be modulated (that is, intermittently opened) with a duty cycle and frequency based on the bandwidth of the valve and the pressure differential across the FTIV. Corresponding pressure pulsations upstream and/or downstream of the valve may be estimated by pressure sensors coupled to the fuel tank and/or the canister. In one example, a frequency (or number of pulses) of the valve modulation may be compared to the frequency (or number of pulses) of the pressure pulsation. In response to a frequency ratio of pressure pulses to valve pulses being lower than a threshold, valve degradation may be indicated. In another example, the FTIV may be modulated with a selected frequency and valve degradation may be indicated based on the amplitude of the pressure pulsations at the selected frequency. For example, the pressure pulsation may be filtered, in parallel, through each of a band-pass filter and a notch filter. In response to a difference of the outputs (or amplitudes) from the two filters being lower than a threshold, valve degradation may be indicated. Further, to enable emissions compliance, in response to the indication of valve degradation, a leak detection operation may be disabled during a subsequent engine-on diurnal cycle.
In this way, pressure fluctuations across the FTIV may be correlated with a modulation of the valve to verify valve functionality. By confirming FTIV functionality before checking for systems leaks, emissions compliance may be better ensured.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.